macosx: addons manager: process the callback on the main thread
macosx: addons manager: process the callback on the main thread

Avoids concurrent access to _addons and _displayedAddons.

krichter722/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2,xkfz007/vlc,krichter722/vlc,shyamalschandra/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,krichter722/vlc,vlc-mirror/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc,krichter722/vlc,jomanmuk/vlc-2.2,shyamalschandra/vlc,krichter722/vlc,shyamalschandra/vlc,xkfz007/vlc,vlc-mirror/vlc,xkfz007/vlc,jomanmuk/vlc-2.2,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,vlc-mirror/vlc,xkfz007/vlc,shyamalschandra/vlc,vlc-mirror/vlc